The present invention relates to an improvement of a circuit for detecting an index of a magnetic recording medium and outputting a predetermined index signal in, for example, a floppy disk apparatus.
In prior 5-inch or 8-inch floppy disk apparatus, an index hole is formed in magnetic recording medium (hereinafter, referred to as a disk) to define the start point for recording/reproduction. The index signal output apparatus optically detects the index hole and generates an index pulse signal each time the disk rotates once.
On the other hand, in the 3.5-inch floppy disk apparatus which has recently been developed, the positions in the rotating direction of the spindle and disk are unconditional due to the relation of the chucking mechanism. Therefore, a method of detecting the index by detecting the rotating position of the spindle was developed. According to such an index detecting method, a detecting circuit using a magnetic sensor is ordinarily provided.
In this index detecting method, a magnet for the index is attached to a predetermined position of the spindle. The magnetic sensor measures the magnetic field generated by the magnet and outputs a corresponding signal. This signal is compared with a predetermined reference signal, so that a predetermined pulse signal (detection pulse signal) is output.
In the above index detecting circuit, when the ambient temperature changes, the magnetic field generated from the magnet varies, causing the timing of the generation of the detection pulse signal to become unstable. For example, when the ambient temperature rises, the magnetic field of the magnet decreases. The timing when the output of the magnetic sensor reaches a predetermined reference level is delayed. Thus, the generation of the detection pulse signal is also delayed. There is also a drawback due to a deviation in the timing of generation of the index signal from the expected timing.
The time period from after the index is detected until the origin of the data reaches the position of the head varies for every disk drive due to the difference of the attaching position of the magnet, the difference of the attachment of the magnetic sensor, the difference of the structure of the disk apparatus, and the like. On the other hand, the time interval from after the index signal is output until the origin of the data reaches the head position needs to be kept constant. Therefore, the time interval from the detection of the index until the generation of the index pulse signal needs to be adjusted for every disk drive. The conventional disk apparatus, therefore, is provided with a circuit to adjust the timing of generation of the index signal (i.e., index control circuit). This control circuit is conventionally comprised of a monostable multivibrator. The timing of generation of the index pulse signal is adjusted by adjusting a resistance value of a variable resistor for adjustment of a pulse width of an output pulse signal of the multivibrator.
The time interval from the output of the detection pulse signal until the origin of the data reaches the magnetic head position varies for every disk drive. Therefore, in the control circuit using the monostable multivibrator, a variable range of the variable resistor for adjustment of the pulse width needs to be set to a large value. Consequently, the timing of generation of the index pulse signal varies greatly in response to a slight change in value of the resistor for adjustment. Thus, there is a drawback in the use of the conventional control circuit because the timing of the generation of the index pulse signal is likely to vary after the pulse width has been adjusted due to the temperature, humidity, vibration, or the like. In association with this variation, there is also a drawback because the time interval from after the index pulse signal was output until the origin of the data reaches the magnetic head position is unstable.